Echo cancellers are used in telephone or similar communication lines and related communications systems to remove the echo from the voice communication and improve voice quality on a telephone call. The Public Switched Telephone Network (PSTN) often generates echo when electrical energy is reflected at a 2W to 4W conversion point. For example, most telephone local loops use two-wire circuits. Transmission facilities, on the other hand, use four-wire circuits. A 2W to 4W converter is used when converting from two-wire to four-wire circuits, which produces echoes.
The magnitude of the echo depends on how well the 2W to 4W converter is set to match the connected 2W facility. To the extent that the match is imperfect, echo becomes part of the telephone communications. Although tolerable in some telephone systems as long as the delay is short, for example, shorter than about 40 milliseconds, the longer delays are distracting and confusing to a far-end speaker. The echo canceller is used towards each end of a path to cancel echo that otherwise would return to the far-end speaker. Some echo cancellers monitor the signals on the received channel of a four-wire circuit and generate estimates of the actual echoes expected to return over the transmit channel.
It is known that full duplex two-wire telecommunications systems use a single wire-pair (two wires) to transmit and receive data, which typically have overlapping spectra or frequency bands. Echo cancellation is also sometimes used to separate the transmitted and received signals to facilitate full duplex data communications. Sometimes the echo cancellation is performed using a full-duplex to half-duplex conversion circuit, for example, a two-wire to four-wire converter or hybrid.